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The star is slightly smaller and cooler than the Sun, being only a fifth as bright, and has been found to have five planets in orbit around it by a planet-seeking space telescope.

Nasa has simultaneously announced the discovery of another super-Earth in another star system, labelled Kepler-69, in the neighbouring constellation of Cygnus, with two known planets, known as Kepler-69b and Kepler-69c.

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In the first system, two of the planets, labelled Kepler-62e and Kepler-62f, have intrigued planetary scientists because of their likely nature. Computer modelling by researchers at the Harvard-Smithsonian Center for Astrophysics (CfA) indicates that they are both covered by global oceans without any land protruding.

It means that though the planets are theoretically habitable, any life there must be aquatic -- perhaps a kind of alien fish!

Lisa Kaltenegger, of the Max Planck Institute for Astronomy and the CfA, led the study into Kepler-62's strange worlds. She said: "These planets are unlike anything in our solar system. They have endless oceans. "There may be life there, but could it be technology-based like ours? Life on these worlds would be under water with no easy access to metals, to electricity, or fire for metallurgy. "Nonetheless, these worlds will still be beautiful blue planets circling an orange star -- and maybe life's inventiveness to get to a technology stage will surprise us."

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The new study suggests that Kepler-62e is 60 percent larger than the Earth while Kepler-62f is about 40 percent larger. They are too small for the team to be able to measure their masses, but they expect them to be composed of rock and water.

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Kepler-62e will be the warmer world and is likely to be cloudier than our own planet. The more distant Kepler-62f would need the greenhouse effect from plenty of carbon dioxide to warm it enough to host an ocean. If that is not the case, it might become more like an ice-covered snowball.

Harvard astronomer and co-author of the study Dimitar Sasselov said: "Kepler-62e probably has a very cloudy sky and is warm and humid all the way to the polar regions. Kepler-62f would be cooler, but still potentially life-friendly. "The good news is -- the two would exhibit distinctly different colours and make our search for signatures of life easier on such planets in the near future."

Kepler-69c is the super-Earth in the second new planetary system. It is 70 percent larger that Earth and orbits its home star in 242 days. That star is similar to the Sun, being 93 percent as big and 80 percent as bright.

John Grunsfeld, associate administrator of Nasa's Science Mission Directorate, said: "The discovery of these rocky planets in the habitable zone brings us a bit closer to finding a place like home. It is only a matter of time before we know if the galaxy is home to a multitude of planets like Earth, or if we are a rarity."

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To date, Kepler has discovered 115 confirmed exoplanets, with 2,740 more suspected, since its launch in March 2009. It does so by staring constantly at around 160,000 stars in one small region of the Milky Way in the constellations of Cygnus and Lyra.

The space telescope is watching for any dip in a star's light that may indicate a transit -- a planet passing in front of it. By measuring the effects on the star's light that the transit has, scientists can learn a surprising amount about the planet itself.

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